1. Field of the Invention
This invention relates to a method of processing a radiographic image, and more particularly to a method of improving the signal-to-noise ratio (hereinafter referred to simply as S/N ratio) of a radiographic image for diagnostic purposes to improve the diagnostic efficiency and accuracy thereof.
2. Description of the Prior Art
The radiographic image for diagnostic purposes is a visible image recorded on a radiographic film by use of intensifying screens which represent the difference in X-ray absorption of an object to be diagnosed in the difference in optical density. The radiographic image is subjected to the analysis and diagnosis by doctors or radiologists.
When the radiographic image is used for the purpose of diagnosis, it is required to detect the very minute difference in X-ray absorption of the object to be diagnosed. The ability of detecting the minute difference in X-ray absorption is represented by contrast detecting power of the radiographic image recording system. Since the difference in X-ray absorption between the various parts of the object to be diagnosed like a human body is very small, the contrast detecting power is usually insufficient due to various kinds of noise inherent in the radiographic image recording system.
Heretofore, as the causes of lowering the contrast detecting power in the radiographic image for diagnostic purposes, there have been mainly known the noise caused by the X-ray quantum mottle and the scattering X-rays from the object. Therefore, it is expected that the contrast detecting power can be increased by removing the above two causes. According to the investigations conducted by the present inventors, however, it has been proved that the contrast detecting power cannot be increased simply by removing said two causes. In other words, even if a radiographic image was made by exposing a radiographic film to X-rays having dose of 10 to 100 times as large as that of the X-rays used in the normal radiographic image recording step for reducing the X-ray quantum mottle, the contrast detecting power was not improved materially through the noise due to the X-ray quantum mottle was markedly reduced as compared with the conventional radiographic image. Further, even if a radiographic image was made by use of a slit recording method as disclosed in Japanese unexamined Patent Publication No. 54(1979)-121043 in order to reduce the amount of scattering X-rays, the contrast detecting power was not improved substantially.